A new generation of engines
Revolution within sight: CCS process promises potential for new era of engines
Diesel and gasoline engines meld into one: CCS combustion process for the world of tomorrow
The best of the gasoline and diesel worlds unified in one engine? Sounds like illusion and revolution. But it is feasible: Volkswagen is working intensively on trials for just such an engine system, which could become reality within the next decade. The name of this new engine technology: CCS. These code letters stand for “Combined Com*bustion System”, and they might also possibly stand for the engine of the future. And it can already be experienced in Touran prototypes.
Innovative mixing process
The decisive breakthrough in CCS technology is the in*novative mixing process in which the principles of direct-injection gasoline (TSI) and direct-injection diesel (TDI) meld together. Volkswagen is exploiting its many years of broad-based experience by its brands in both gasoline and diesel areas, and is unifying their strengths. “The gasoline engine contributes the homogeneous fuel-air mixture and low emissions to the CSS process, while the diesel contributes its self-ignition and low fuel consumption”, explains Dr. Wolfgang Steiger, Director of Powertrain Research at Volkswagen Corporate Research. A look at the TSI and TDI worlds clarifies what is meant here:
In gasoline engines mixing occurs as described below based on special properties of the fuels. In the case of direct injection TSI engines from Volkswagen the liquid fuel is injected in the intake stroke, while the piston moves downward. As soon as it reverses its direction in the compression stroke it compresses the mixture and heats it. When the piston reaches “top dead center” (TDC), the fuel is evaporated and uniformly (homogeneously) distributed; a short time later the spark plug ignites the gas cloud. In the diesel engine a considerably different combustion process occurs. Here the fuel is injected when the piston is near TDC, and the air in the combustion chamber has been significantly compressed and heated. The diesel fuel then self-ignites in the hot air. In modern diesel engines combustion frequently does not occur until relatively far after TDC; this especially reduces NOx emissions but at the same time increases fuel consumption.
The world between diesel and gasoline engines
The CCS process lies midway between diesel and gasoline principles. In homogeneous operation CCS engine injection already begins while the piston is traveling upward and the air compresses. Using common rail injectors taken from diesel engines the injection process can be distributed to different cycles and can be metered precisely.
While the piston continues its upward travel, the fuel and air are compressed and heated; the fuel evaporates to form a largely homogeneous gas cloud comparable to that of the TSI engine. In homogeneous mode, combustion is initiated as shortly as possible after TDC, without requiring an external spark (analogous to the diesel). Theoretically the mixture ignites at an infinite number of points simultaneously. Quasi homogeneous combustion near TDC makes it possible to achieve both low emissions and even greater fuel economy compared to the already economical TDI.
Exhaust gas recirculation eliminates nitrogen oxides
The CCS engine operates with a very high gas recirculation rate (EGR rate). The oxygen-poor, recirculated exhaust gas makes several important contributions. It ensures that combustion is not initiated too early and that temperatures or hot spots do not develop that would generate nitrogen oxides. In the CCS process with its uniform combustion there are practically no zones of rich mixture in the combustion chamber. Therefore (in contrast to the diesel) hardly any soot appears, which otherwise would be the shortcoming of high AGR rates.
A first research engine based on a two-liter TDI in the Touran prototype that is currently in an early developmental stage already realizes five percent fuel savings compared to a conventional diesel utilizing an optimized fuel, and it simultaneously reduces NOx and soot emissions significantly.
SynFuel and SunFuel® as replacements for crude oil
The basic precondition for full implementation of the CCS combustion process is a new designer fuel. It helps to achieve homogeneous combustion. Fully aware of the key role played by the fuel issue, engineers at the Volkswagen corporation have been working intensively in this area for many years. The focus of research is on fuels obtained synthetically from natural gas (SynFuel) or biomass (SunFuel®). SynFuel and SunFuel® are free of sulfur and aromatic compounds, and this by itself drastically reduces raw emissions. Within certain constraints their compositions and therefore their properties, especially boiling temperatures and cetane ratings, can be freely defined in the design, and the fuels can be produced in high quality, reliably and reproducibly. In principle, this makes the two fuels ideally suited for use in the CCS engine (see also detailed discussion of the separate topic of “synthetic fuels”). They are energy sources of the immediate future. That these fuels will also drive the combustion engine of the future – “the CCS” – is almost guaranteed. As is so often the case, it is just a matter of time.
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Originally Posted by GTTDI
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